We explore the feeding habits of two populations of the lizard Aspidoscelis lineattissima from two localities (Cocinas = island and Xametla = mainland) under the hypothesis that in an insular environment, there is less selection of food, because it is a relatively comfortable environment to forage due to the presence of few competitors and predators. We also examine the predator–prey size relationship in this lizard species with the intention of distinguishing possible ontogenetic changes in the choice of prey sizes in the two localities. In stomach contents of the lizards from mainland, a total of 2,616 prey items were recorded, while on the island, 1,357 prey items were found. This number of prey was divided into 18 categories for the mainland and 20 for the island. In general terms, the most important prey in the diet of lizards from both localities were Isoptera, Coleoptera (adult and larvae), Araneae, Orthoptera, Hymenoptera (ants and others), Blattodea, and Diptera. Island lizards presented a greater amount of plant material in their stomach contents. The breadth of the food niche was greater on the island than on the mainland. Although statistically there were no differences regarding food overlap between localities, the highest overlap values between groups (age classes and sexes) were found on the mainland. We found a positive relationship between morphological variables of the lizards (snout–vent length, head width, and head length) with the maximum and mean values of the prey volume in both localities. This reveals an ontogenetic change that indicates that as lizards grow, they add a greater quantity of large prey to their diet and at the same time eat smaller prey. These results showed that the diet of the lizards from both localities was similar to other species of the genus. The feeding habits of a species in contrasting environments is evidence that is helpful for understanding the foraging patterns more clearly, and the possible connection with its life history characteristics, shedding more light on the hypothesis of niche amplitude and the optimal foraging theory.